1. Field of the Invention
The present invention relates to a perovskite oxide, a process for producing the perovskite oxide, a ferroelectric compound containing the perovskite oxide, a piezoelectric body, a piezoelectric device using the piezoelectric body, and a liquid discharge device using the piezoelectric body.
2. Description of the Related Art
Currently, piezoelectric devices constituted by a piezoelectric body and electrodes are used as, for example, actuators installed in inkjet recording heads. In the piezoelectric devices, the piezoelectric body expands and contracts according to increase and decrease in the strength of an electric field applied from the electrodes to the piezoelectric body in a predetermined direction. For example, perovskite oxides such as PZT (lead titanate zirconate) are known as materials suitable for the piezoelectric body. Such materials are ferroelectric materials which exhibit spontaneous polarization even when no electric field is applied. The piezoelectric materials are reported to exhibit high piezoelectric performance at and near the morphotropic phase boundary (MPB).
PZT is a solid solution of PbTiO3 (PT) and PbZrO3 (PZ). FIG. 12 is a phase diagram of PZT with respect to the temperature and the mole fraction of titanium (i.e., the mole fraction of PbTiO3) in PZT. The phase diagram of FIG. 12 is cited from “Landolt-Bernstein: Numeral Data and Function Relationships in Science and Technology, New Series,” Group III: Crystal and Solid State Physics, Vol. 16, edited by K. H. Hellwege and A. M. Hellwege, Springer-Verlag Berlin-Heidelberg-New York, 1981, p. 426, FIG. 728. In FIG. 12, FT denotes the tetragonal phase, and FR denotes the rhombohedral phase.
PZT tends to form tetragonal crystals when the Ti composition is high, and rhombohedral crystals when the Zr composition is high. When the molar compositions of Ti and Zr are approximately identical, the MPB composition is achieved. For example, the mole ratio of Zr to Ti of 52/48, which is near to the MPB composition, is preferable. The textbooks on the piezoelectric ceramic materials teach that the crystal structures become unstable and the piezoelectric performance becomes highest at and near the MPB. Conventionally, PZT has been reported to form pseudocubic crystals at and near the MPB. However, details of the nanostructure of PZT are unknown.
In the above circumstances, the Japanese Unexamined Patent Publication No. 2006-036578 (hereinafter referred to as JP2006-036578A) reports that sintered bodies of PZT-based ceramic materials such as Pb(Ti, Zr, Nb)O3 are formed of two-phase mixed crystals of tetragonal crystals and rhombohedral crystals at and near the MPB. (See, for example, claim 9 in JP2006-036578A.) Further, JP2006-036578A discloses that it is possible to desirably design the composition on the basis of the relationship between the piezoelectric coefficient and the phase fractions of the tetragonal and rhombohedral phases. (See, for example, Table 1, FIG. 4, and the paragraph 0027 in JP2006-036578A.)
According to the technique disclosed in JP2006-036578A, it is possible to prepare samples of perovskite oxides each constituted by a plurality of predetermined elements with different mole fractions, obtain the phase fractions of the tetragonal and rhombohedral phases in each sample by X-ray diffraction and Rietveld analysis, obtain the piezoelectric coefficient of each sample, and determine the composition on the basis of the relationship between the obtained phase fractions and the piezoelectric coefficient. However, according to the above technique, it is necessary to search for desirable composition by performing an experiment every time the constituent elements of the sample are changed, so that the material design cannot be efficiently made by the technique.
Further, currently, the public interest in the environmental load is increasing, and demands for lead-free piezoelectric films are increasing. However, JP2006-036578A does not disclose application of the technique disclosed in JP2006-036578A to the lead-free material.
In addition, S. Yokoyama et al., “Compositional Dependence of Electrical Properties of Highly (100)-/(001)-Oriented Pb(Zr, Ti)O3 Thick Films Prepared on Si Substrates by Metalorganic Chemical Vapor Deposition”, Japanese Journal of Applied Physics, Vol. 42, 2003, pp. 5922-5926 also report that PZT films are formed of two-phase mixed crystals of tetragonal crystals and rhombohedral crystals at and near the MPB. (See, for example, FIG. 2(b) in the Yokoyama reference.) However, many aspects of the piezoelectric mechanism and the crystal structure at and near the MPB are still unknown.
Further, one of the present inventors (Yukio Sakashita) and colleagues belonging to the present assignee have proposed in the Japanese Unexamined Patent Publication No. 2007-116091 (which is hereinafter referred to as JP2007-116091A) a piezoelectric device using a piezoelectric body which contains regions in a first ferroelectric phase having crystal orientation. In the piezoelectric body, the phase of at least a portion of the above regions transitions from the first ferroelectric phase corresponding to a first crystal system to a second ferroelectric phase corresponding to a second crystal system different from the first crystal system when an electric field is applied to the piezoelectric body.
In the above piezoelectric device, it is possible to achieve a volume change caused by a change in the crystal structure associated with the phase transition from the first ferroelectric phase. In addition, since the piezoelectric effect works in both of the first ferroelectric phase (before the phase transition) and the second ferroelectric phase (after the phase transition), the piezoelectric device disclosed in JP2007-116091A exhibits great displacement when the electric field is applied to the piezoelectric body.
Further, JP2007-116091A reports that the engineered-domain effect and the like increase the distortion amount (displacement) when the direction along which the electric field is applied to the piezoelectric body is different from the orientation of the spontaneous polarization axis in the ferroelectric phase before the phase transition, and is preferably approximately identical to the orientation of the spontaneous polarization axis in the ferroelectric phase after the phase transition.